The right thread
If screw jacks no longer work, this may be due to the surface quality of the spindles, contamination, the installation position or, in particular, the choice of lubricant – says Hans Gereke-Bornemann, Managing Director of Bornemann Gewindetechnik, who also has a solution for this.
He knows what he is talking about, as Bornemann-Gewindetechnik has now accumulated over 25 years of drive technology experience: “Today, we mainly manufacture threaded spindles according to German and international standards, special and trapezoidal threaded spindles, screw conveyors, spindle nuts, as well as turned parts and milled parts – on around 3,800 m2 with 40 employees.” But what points currently need to be considered when selecting the right thread, especially for lifting systems? “We recommend that our customers pay very close attention to the thread surface and the choice of lubricant,” says Hans Gereke-Bornemann, pointing out the first ‘hot spots’. Because when Bornemann reports problems with screw jack systems, one of the following factors is usually part of the cause: the surface quality of the screws, contamination, the installation position of the system or, in particular, the choice of lubricant.
In many cases, however, the problem is a complex mixture of several of these factors. “It is therefore important to give sufficient consideration to each of the factors in the design – otherwise movement threads often quickly reach their limits”. This is because all motion threads, including worm drives, are so-called sliding thrust drives. Due to this type of drive, a stick-slip effect is possible at any time, especially at low speeds and without sufficient lubrication. “Combined with an incorrect material pairing, such inadequate lubrication conditions can even lead to micro or cold welding of entire lifting systems, known as ‘seizing’ of the nuts.”
In Hans Gereke-Bornemann’s experience, the material pairing of the spindle-nut system therefore plays a decisive role. “With material pairings of gray cast iron, for example ductile cast iron and steel, the iron content in the materials can promote cold welding. With a combination of bronze and steel, cold welding can be virtually ruled out.” However, there are other ways to minimize these sometimes considerable risks: “With worm drives, care is taken to ensure that the contact pattern in the middle of the rolling circle has a lenticular shape. This is the only way to prevent premature oil breakage. If both profiles were to wear optimally, the oil film would break off immediately.”
The whirling process has further potential for the production of threads. “This achieves an effect that has similar properties to the worm gears described above. Many users of trapezoidal threads are hardly aware of this fact. The whirling technique produces a geometric deviation from the ideal screw shape both on the thread flanks and in the base: “This is the enveloping circle cut, which, depending on the cutting frequency, consists of many polygons that only deviate from the ideal shape in the µm range,” explains Hans Gereke-Bornemann. This means that there is no abrupt oil breakage in these very small clearances, as the oil cannot be displaced or wiped off so easily. The geometric deviation from the ideal screw shape almost automatically forms microscopically small lubrication pockets on the flank surface. “This significantly reduces the risk of lubricant film breakage,” promises Hans Gereke-Bornemann.
But how does the favored whirling process compare to thread rolling? “Rolled threaded spindles certainly have manufacturing advantages in many cases, but certainly not with lifting spindle drives using trapezoidal screw drives, as no high-quality materials, let alone stainless steels, can be used with this process. In addition, individual parts with special profiles cannot be produced economically, as the tools are so expensive that only mass-produced parts are really worthwhile. “Furthermore, rolled threaded spindles are nowhere near as precise as we can achieve with the whirling process.”
However, Hans Gereke-Bornemann would recommend rolled thread profiles to any customer for low-end applications where precision, lubrication, material and any special profiles are not important: “Of course, the material of rolled spindles is stronger due to the cold forming process. However, the constant comparison with the supposedly higher load ratings is misleading because the nut is the weak point in this system and not the spindle; the nut, which is usually made of bronze, actually wears faster with a rolled spindle than with a whirled spindle.”
From Harald Klieber